1. Field of the Invention
The present invention relates to an optical deflector which is incorporated in an optical unit of a laser beam printer or the like and which deflects and guides a laser beam emitted by a laser diode to an object to be scanned.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner or an optical deflector and then to a photosensitive body, i.e., an object to be scanned. The photosensitive body is scanned with the laser beam at a constant speed. When guided from the laser diode to the photosensitive body, the laser beam passes through a group of focusing lenses. The focusing lenses alter the laser beam to have a desirable cross sectional shape, and then direct the laser beam to a predetermined position on the surface of the photosensitive body.
In this type of optical unit, the optical deflector includes a polygonal mirror having a plurality of non-planar mirror surfaces, for example, four or eight mirror surfaces. When the polygonal mirror is rotated, the optical deflector reflects the laser beam toward the photosensitive body at a nonuniform angular velocity.
With respect to the polygonal mirror of the above-mentioned type, it is known that the width of the line to be scanned with the laser beam that is deflected by one mirror surface, i.e., the width of the scanned line on the photosensitive body in the main scanning direction during one scan, is very narrow. The width of the line to be scanned is in the range of several tens of .mu.m to 100 .mu.m or so. Therefore the polygonal mirror must be coupled to the shaft of the motor very accurately. To do this the polygonal mirror is first pressed against a base member (a fixing member) of the shaft of the motor and is then secured thereto by use of a leaf spring (a pressing member).
However, the leaf spring is hard to design because the polygonal mirror must be pressed very strongly against the base member of the motor shaft. In addition, because the leaf spring is kept strongly pressed at all times, the force it produces may vary with time. The leaf spring must be designed in accordance with such a force variation, as well. These problems result in an increase in the manufacturing cost of the optical deflector.